Most direct conversion radio frequency transceivers require baseband correction for I/Q mismatch. The transmitter up-converts a quadrature baseband signal to a radio frequency quadrature signal. More specifically, the transmitter includes a first mixer that mixes the in-phase component of the quadrature baseband signal with an in-phase component of a local oscillator signal to up convert the in-phase component of the baseband signal to the in-phase component of the radio frequency signal. Similarly, the transmitter includes a second mixer that mixes the quadrature component of the quadrature baseband signal with a quadrature component of a local oscillator signal to up convert the quadrature component of the baseband signal to the quadrature component of the radio frequency signal.
However, the gain of the first mixer may not be matched to the gain of the second mixer. As a result, a gain error is introduced into the radio frequency signal. In addition, the in-phase component and the quadrature component of the local oscillator signal used by the mixers for up-conversion may not be ninety degrees out-of-phase. As a result, a phase error, or quadrature error, is introduced into the radio frequency signal. Combined, the gain and quadrature errors form an I/Q mismatch of the transmitter. In a similar fashion, the receiver of the direct conversion receiver has an I/Q mismatch. It is desirable to correct the I/Q mismatch of the transmitter and/or receiver at baseband in order to provide improved performance.
Generally, baseband correction of the I/Q mismatch of either the transmitter or the receiver is performed based on a 2×2 distortion matrix defining a relationship between the actual I and Q components having been distorted by the I/Q mismatch of either the transmitter or receiver and the ideal I and Q components. However, the distortion matrix is not easily extracted, or calibrated. This is especially true for transceivers wherein the baseband processor, the transmitter, and the receiver are integrated into a single integrated circuit or module. In this case, the baseband inputs of the transmitter and receiver are not accessible by an external device. Thus, for transceivers operating according to IEEE 802.11a or 802.11g, the transmitter may only have Orthogonal Frequency Division Multiplexing (OFDM) packets available.
In the past, calibration was performed by providing a known complex tone to the baseband inputs of the transmitter. Then, the I/Q mismatch, and specifically the distortion matrix, of the transmitter was computed based the known complex tone and the output of the transmitter. The receiver was calibrated in a similar fashion. However, since these special test signals are not always available or economic, for many direct conversion transceivers, this method of calibrating the correction matrix is no longer viable.
Thus, there remains a need for a system and method for providing economic baseband correction of an I/Q mismatch of a direct conversion radio frequency transceiver.